Question

I’m trying to define a trait that describes operators based on other operator. Something like this:

trait LessThanComparable[T] {
    def < (that: T) : Boolean

    def > (that: T) = that < this
}

Then I use it:

class Example(val x : Int) extends LessThanComparable[Example] {
    def < (that: Example) = x < that.x
}

But I get this: value < is not a member of type parameter T

How can I say that that and this are of the same Type? Or am I trying something impossible?

Answer 1

I think this is what you want:

trait LessThanComparable[T <: LessThanComparable[T]] { this: T =>
  def <(that: T): Boolean

  def >(that: T) = that < this
}

class Example(val x: Int) extends LessThanComparable[Example] {
  def <(that: Example) = x < that.x
}

In order to be able to say that < this, two things must hold.

1. that must have a < method that accepts a T, or in other words, that must be a LessThanComparable[T]. We can ensure this by saying that T must be a subclass of LessThanComparable[T], or T <: LessThanComparable[T].

2. this must be a T. We can ensure this by using a self type this: T =>.

So then,

val a = new Example(5)
val b = new Example(4)

println(a < b)  // false
println(a > b)  // true
println(b < a)  // true
println(b > a)  // false